Field of the Invention
The present invention relates to a transducer, a method for manufacturing the transducer, and an object information acquiring apparatus and, in particular, to a capacitive transducer used as an ultrasonic transducer, a method for manufacturing the capacitive transducer, and an object information acquiring apparatus.
Description of the Related Art
Capacitive Micromachined Ultrasonic Transducers (CMUTs), which are one type of capacitive transducer using a micromachining technology, have been studied for a replacement of a piezoelectric element. Such capacitive transducers can transmit and receive ultrasonic waves using the vibration of a vibrating membrane.
Each of elements of a CMUT includes a plurality of cells. A gap of each of the cells can be formed by etching a sacrifice layer through an etching hole. Thereafter, the etching hole is filled up and, thus, is sealed. Japanese Patent Laid-Open No. 2008-98697 describes a technique for forming a gap of a cell by forming a plurality of holes in the cell and performing etching through the holes so that a gap of the cell is formed. In addition, each of the gaps of the cells is sealed, and the gaps do not communicate with one another. In contrast, in Japanese Patent Laid-Open No. 2011-254281, a single etching hole is formed for a plurality of cells. Etching liquid enters the plurality of neighboring etching holes. At that time, the etching holes are disposed so that the front lines of progressing etching for the cells do not intersect in a region under a vibrating membrane. In this manner, etching residue does not remain in the gaps.
If, as described in Japanese Patent Laid-Open No. 2008-98697, the sacrifice layer is removed by forming an etching hole for each of the cells, it is difficult to arrange the cells in high density, since a plurality of etching holes are present. Accordingly, as compared with transducers including a plurality of cells in high density, the transmission efficiency and reception sensitivity, that is, the conversion efficiency of the transducer decreases.
In contrast, in Japanese Patent Laid-Open No. 2011-254281, a plurality of cells share a single etching hole and, thus, the cells can be arranged in high density. A downside of the technique of Japanese Patent Laid-Open No. 2011-254281 is that the gaps of all of the cells are connected through an etching channel. If a seal failure occurs in one of the etching holes, the transmission efficiency and reception sensitivity of the element significantly decrease. In particular, when the transducer is used in liquid, the liquid may enter the gap and, thus, the transmission efficiency and reception sensitivity of the element may further decrease.